Reduced response of pesticide peaks in GCMS

[little_brad]

Hey there,

I am currently working on a project which uses Quechers for analysis of multi-residue pesticides in vegetation. This involves running acetonitrile extracts on GCMS, i am using splitless injection.

I decided to prepare some standards in acetonitrile and run them to see how they compare to the current set of standards for our old method (in DCM). I noticed that the responses are much lower than the original standards in DCM and some of the peaks are also not as good (tailing).

I installed a guard column which improved peak shape and intensity somewhat but it still seems as though the reponse of the peaks is lowered and i was wondering whether this is dude to the solvent or something else which i am missing and if there is a way to increase the response.

The system i am using is an Agilent 6890/5973, column DB5-ms, guard column is phenylmethyl deactivated.

For the standards in DCM i use initial oven temp 40°C, 2µL splitless injection, 280°C injector port, 6.8psi head pressure.

For standards in acetonitrile i use initial oven temp 75°C, 1µL splitless injection, 250°C injector port, 10psi head pressure (obviously lower injection decreases response however even taking this into account, responses still seem a bit low, also had to use these parameters to avoid backflash from solvent volume expansion)

Any clues on why I am obtaining a lower response for the pesticide peaks when going from DCM - Acetonitrile as solvent? Also if it possible for me to use a 2µL injection somehow by changing parameters that would be great so I can increase my LOQ.

If you require any other method parameters or info let me know!

Cheers

Brad

[JI2002]

Why did you use different initial oven temperature? Try 40C for acetonitrile and see if it helps.

[little_brad]

75°C is stated in the Quechers method so i followed that.

the boiling point of acetonitrile is 82°C, wouldn't having such a low oven starting temp maybe make the solvent condense in the column or something?

[Don_Hilton]

Sometimes you want the solvent to condense in the column. You get a solvent focusing effect as the solvent evaporates into a narrower and narrower band - pulling the analytes into a narrow band on the column. You will notice that peaks that lead the solvent peak on such an injection become broadened and peaks that follow the solvent band are narrowed. But, the solvent and the column must be compatible. If the solvent will not wet the stationary phase (like methanol on a DB-5 column) it does not form a nice band and peak shapes are terrible until you are injecting onto a column warm enough to keep the solvent from condensing.

When I develop a method, I have a habit of making injections below the boiling point of the solvent and then ramping the column temperature up as fast as I can and still get good separation. The solvent effect is stronger on peaks close to the solvent peak. A fast ramp keeps peaks narrow - but does shove them together.

40 degrees does seem a bit cool for acetonitrile. You want it to condense on injection, evaporate at the start of the run - and be gone. You are not going to be doing much useful (other than focusing the analytes that will focus) until the solvent clears the column.

[JI2002]

The rule of thumb for oven initial temperature is 20C below the bp of the solvent. I think the oven temperature plays a role in the lower response for acetonitrile. To try 40C doesn't mean to use 40C for the method.

[little_brad]

thanks guys, yeah the first few peak are the main issue, dichlorvos especially as it is the first peak that elutes, i tried increasing the injector temp to 280°C and the dichlorvos peak is now nice and sharp!

still have the same issue that overall all compounds have a lowered response...... all peaks are well resolved and the calibration is linear so i guess it doesn't matter that much

[jh1]

Any clues on why I am obtaining a lower response for the pesticide peaks when going from DCM - Acetonitrile as solvent? Also if it possible for me to use a 2µL injection somehow by changing parameters that would be great so I can increase my LOQ.

If you require any other method parameters or info let me know!

Cheers

Brad

To run 2uL injections try a tapered liner with greater than >2ul capacity running pulsed splitless.

[k.m.seenivasan]

Hi,
I am working in pesticide analysis in egg. I'm using florisil column cleanup to remove fat. Now i want to swithover to SPE. I got free Florisil SPE cartridge along with J& W column purchase. But prior to use the SPE iwant to know the combination of solvents for this high fat sample. If you are working with high fat samples, can you share the information?

Regards

seeni